Strokes fall into two major categories: ischemic strokes, caused by blocked blood flow to the brain, and hemorrhagic strokes, caused by bleeding in or around the brain. Ischemic strokes account for roughly 87% of all strokes, making them far more common. Within each category, several distinct subtypes exist, each with different causes, risk profiles, and treatment approaches.
Ischemic Stroke
An ischemic stroke happens when a blood vessel supplying the brain becomes blocked, cutting off oxygen and nutrients to brain tissue. Within minutes, brain cells in the affected area begin to die. The damage starts with an energy crisis: without blood flow, cells can’t maintain their normal chemistry, and a toxic cascade of calcium buildup and inflammation spreads outward from the blocked area.
There are two main ways this blockage occurs.
Thrombotic Stroke
A thrombotic stroke develops when a blood clot forms directly inside an artery that supplies the brain. This typically happens in arteries already narrowed by a buildup of fatty deposits along the vessel walls. It’s more common in older adults, particularly those with high cholesterol or diabetes, because those conditions accelerate the gradual narrowing process over years. The clot forms at the site of the narrowing, eventually blocking enough blood flow to cause a stroke.
Embolic Stroke
An embolic stroke starts somewhere else in the body, most often the heart. A blood clot or fragment of debris breaks loose, travels through the bloodstream, and lodges in a brain artery too small for it to pass through. These strokes tend to strike suddenly and without warning. About 15% of embolic strokes occur in people with atrial fibrillation, a condition where the upper chambers of the heart quiver instead of beating effectively, allowing blood to pool and clot. Heart disease and prior heart surgery also raise the risk.
Hemorrhagic Stroke
A hemorrhagic stroke occurs when a blood vessel in or around the brain ruptures, spilling blood into surrounding tissue. The leaked blood pools into a mass called a hematoma, which presses on nearby brain structures and disrupts their function within hours. The damage doesn’t stop there. Compounds released from the pooled blood, including iron and clotting proteins, trigger a wave of secondary injury: inflammation, swelling, and breakdown of the protective barrier between the bloodstream and the brain.
Intracerebral Hemorrhage
Intracerebral hemorrhage means bleeding directly into the brain tissue itself. High blood pressure is the most common trigger. Over time, chronic hypertension weakens small arteries deep in the brain until one gives way. Other causes include brain tumors, abnormal tangles of blood vessels (called arteriovenous malformations), and the use of blood-thinning medications. A condition called cerebral amyloid angiopathy, where a protein gradually weakens blood vessel walls, is a significant cause of this type of bleeding in older adults.
Subarachnoid Hemorrhage
Subarachnoid hemorrhage involves bleeding into the fluid-filled space surrounding the brain rather than into the brain tissue directly. The most common cause is a ruptured brain aneurysm, a weak, balloon-like bulge in an artery wall that eventually bursts. Arteriovenous malformations can also be responsible. This type of hemorrhagic stroke often produces a sudden, extraordinarily severe headache described as the worst headache of a person’s life, along with neck stiffness, nausea, and sometimes loss of consciousness.
Transient Ischemic Attack (TIA)
A transient ischemic attack, sometimes called a mini-stroke, involves a temporary blockage that resolves on its own. Most TIA symptoms disappear within an hour, though they can last up to 24 hours. The symptoms mirror those of a full ischemic stroke: sudden numbness, confusion, trouble speaking, or vision changes. The critical difference is that a TIA doesn’t cause permanent brain damage.
That doesn’t make it harmless. A TIA is a warning sign that the conditions for a full stroke are already in place: a narrowed artery, a tendency to form clots, or an irregular heartbeat sending small clots toward the brain. The risk of a major stroke in the days and weeks following a TIA is significant, which is why it demands the same urgency as a completed stroke.
Cryptogenic Stroke
Sometimes an ischemic stroke occurs and doctors can’t identify a clear cause even after ruling out the usual suspects: large-artery narrowing, small-vessel disease, atrial fibrillation, clotting disorders, and artery tears. These strokes are labeled cryptogenic, meaning “of hidden origin.”
One of the most common hidden culprits is a patent foramen ovale, or PFO, a small hole between the upper chambers of the heart that never fully closed after birth. In the general population, roughly one in four people has a PFO without knowing it. But among people who’ve had a cryptogenic stroke, 40% to 50% have one. The hole can allow a clot that formed in a vein to slip through to the arterial side and travel to the brain, bypassing the lungs where it would normally be filtered out. When a PFO is combined with a floppy flap of tissue in the heart wall (called an atrial septal aneurysm), the risk of a first stroke and especially a recurrent stroke rises substantially.
Brainstem Stroke
The brainstem is a small structure, only about half an inch in diameter, that sits at the base of the brain and controls some of the body’s most essential functions: consciousness, breathing, blood pressure, and the relay of all motor signals between the brain and the body. A stroke here, whether ischemic or hemorrhagic, can look very different from strokes in other brain regions.
The hallmark of most strokes is one-sided weakness, but a brainstem stroke may instead cause vertigo, severe imbalance, double vision, slurred speech, or decreased consciousness without any obvious limb weakness. This can make it harder to recognize quickly. In severe cases, a brainstem stroke can result in locked-in syndrome, where a person remains fully conscious and aware but can move only their eyes. Because the brainstem governs breathing and heart function, strokes in this location can be life-threatening even when the area of damage is small.
How Treatment Differs by Type
The distinction between ischemic and hemorrhagic strokes isn’t just academic. It fundamentally changes what happens in the emergency room, because the treatments are essentially opposite. Giving a clot-dissolving drug to someone with a bleeding stroke could be fatal, which is why brain imaging is the first step after arrival.
For ischemic strokes, the primary goal is restoring blood flow as fast as possible. Clot-dissolving medication can be given intravenously within 4.5 hours of when symptoms started or when the person was last known to be well. For strokes caused by a blockage in a large brain artery, a procedure called mechanical thrombectomy can physically retrieve the clot using a catheter threaded up from the groin. Recent evidence shows this procedure can be effective even in an extended window up to 24 hours after symptom onset in selected patients, a major shift from earlier, stricter time limits.
Hemorrhagic strokes require a different approach. If the bleeding stems from a ruptured aneurysm, a procedure called endovascular coiling can seal it off. A catheter is guided from a groin artery up to the brain, and tiny platinum coils, some thinner than a human hair, are packed into the aneurysm to block further blood flow into it. This can be done under general anesthesia and is used both to prevent rupture in high-risk aneurysms and to treat those that have already burst. For arteriovenous malformations, similar catheter-based techniques can close off the abnormal vessels. When bleeding causes dangerous pressure buildup inside the skull, surgery to drain the collected blood or relieve pressure may be necessary.
Regardless of type, every minute matters. Brain tissue deprived of oxygen dies quickly, and expanding bleeding causes compounding damage. The phrase “time is brain” exists for a reason: faster treatment consistently leads to better outcomes and less permanent disability.